A new high strength and high tolerance-resistance Al-Li alloy
YANG Shou-jie(杨守杰), LU Zheng(陆 政),
DAI Sheng-long(戴圣龙), HAN Ya-fang(韩雅芳), YAN Ming-gao(颜鸣皋)
Beijing Institute of Aeronautical Materials, Beijing 100095, China
Received 28 July 2006; accepted 15 September 2006
Abstract: In order to develop a new high strength and high tolerance-resistance Al-Li alloy which can be used in aerospace industry, the effects of microalloying elements such as Mg, Ag, Mn and Zn on the mechanical properties of Al-Cu-Li alloys were studied. The results show that the strengthening effects of Mg+Ag and Mg+Zn additions are higher than those of the individual Mg, Ag or Zn addition. The element Mn can also bring some extent strengthening effects on the alloys, but it has nothing to do with the other microalloying elements present or not. Finally, a new Al-Li alloy with Mg+Zn+Mn additions was developed, which possesses high strength and high tolerance-resistance promising properties for aerospace applications.
Key words: Al-Cu-Li alloys; microalloying; mechanical properties; strengthening phase
1 Introduction
Since 1990’s, the third generation Al-Li alloys have been used for aerospace industry, such as 2195, 2196, 2097 and 2197[1-3]. Those alloys all belong to Al-Cu-Li system alloys. The alloys such as 2195 and 2196 possess high strength but low toughness, and the addition of Ag will increase cost. The alloys such as 2097 and 2197 without Ag addition possess high toughness but low strength. In the Al-Cu-Li alloys, the T1 phase is considered one of the most effective strengthening phases. In order to stimulate a fine distribution of T1 precipitates in the artificial aging, small additions of Mg and Ag are utilized for serving as nucleation aids[4-6]. Recently, GREGSON and FLOWER[7] have reported that 8090 Al-Li alloys with additions of zinc, magnesium and copper may produce a homogeneous distribution of the S′ phase, which further improves the strength and also helps to disperse the coplanar slip. In the Al-Zn-Mg-Cu alloys, SODERGREN and LLOYD[8] found that the addition of 0.9%Li (mass fraction) to a 7029 alloy will inhibit the formation of GP zones and metastable η′ (MgZn2) phase, and after prolonged aging or aging at higher temperature the T[(Al,Zn)49Mg32] phase is formed. The properties of 7029+0.9%Li become worse. KILMER and STONER[9] also reported that small additions of Zn in 8090 alloy probably alter the precipitation process. So it is very necessary to investigate the effects of Mg, Ag, Mn and Zn on Al-Cu-Li alloys systematically for developing new Al-Li alloys.
The purpose of this paper is just to investigate effects of microalloying elements such as Mg, Ag, Mn and Zn on the mechanical properties of Al-Cu-Li alloys systematically, and to develop a new Al-Li alloy with high strength and high tolerance-resistance for aerospace applications.
2 Experimental
The Al-Cu-Li alloys in this study were prepared from high-purity materials in argon atmosphere and homogenized for 16 h at 500 ℃. These ingots were then extruded at a ratio of 16 to a bar, with 12 mm in diameter, and the new alloy was hot rolled to 30 mm-thick plates. Solution treatment for the samples was carried out at 515℃ for 30 min followed by cold water quenching. After stretched by about 5%, the samples were aged at 175 ℃ for 16 h.
Tensile mechanical property tests were performed on an Instron-4507 test machine at room temperature. Fracture toughness, thermal exposure, SCC resistance and fatigue crack growth propagation were also determined in accordance with standard practice. Optical microscope and transmission electron microscope(TEM) were applied to examine the microstructure of aged samples. TEM experiments were performed at 200 kV using a JEOL-200CX with a double tilt holder.
3 Results and discussion
3.1 Effects of microalloying elements
According to the metastable phase diagrams for the Al-Cu-Li system in Ref.[10], the alloys in the Al+T1 phase field are selected for this study.
Compositions and mechanical properties of the base alloy X1 and the alloys with Mg, Ag or Mg+Ag additions are listed in Table 1. The age-hardening curves of the X11, X12 and X13 alloys under 175 ℃ for 0-36 h are illustrated in Fig.1. It can be seen that the mechanical properties of Al-Cu-Li-Zr system alloys are promoted when the microalloying element Mg or Ag is added. The yield strength of X13 alloy containing Mg and Ag is about 90 MPa higher than that of X1 alloy.
Fig.1 Age-hardening curves of alloys X11, X12 and X13 under 175 ℃
The results in Fig.1 show that the hardness of X11 with Ag is lower than that of X12 and X13 before aging, and at the age-hardening peak (about 20 h) it increases to the comparable value. This indicates that the addition of Mg will accelerate the age-hardening behavior in Al-Cu-Li-Zr system alloys not only just after quenching and aging at room temperature(RT) but also in the artificial aging process.
The role of Mn and Zn in Al-Cu-Li-Zr system is also studied in the present study. The composition and mechanical properties of the base alloy X2 and the alloys with Mn or Zn addition are listed in Table 2. It is found that the additions of Mg+Zn+Mn can bring higher strengthening effects, but reduce the elongation of the Al-Cu-Li-Zr alloy. The individual addition of Zn in alloy M2 without Mg has no good influence on the mechanical properties. The age-hardening curves of M1, M2 and M3 alloys under 175 ℃ for 0-36 h are also illustrated in Fig.2. It is found that the alloy M3 exhibits faster age-hardening behavior and higher hardness than M1 and M2 alloys, which indicates that the addition of Mg will accelerate the age-hardening behavior similar to that of the alloy X13 in Fig.1.
Fig.2 Age-hardening curves of alloys M1, M2 and M3 under 175 ℃
Table 1 Compositions and properties of alloys for investigating effects of Mg and Ag
Table 2 Compositions and properties of alloys for investigating effects of Mn and Zn
In order to determine the optimum content of Zn in Al-Cu-Li-Zr alloys, three alloys Z1, Z2 and Z3 are designed. Compositions and tensile properties are listed in Table 3. It is found that the small addition of Zn can elevate the strength significantly in Z1 alloy. However, the strength cannot increase again and the elongation become worse with increasing the content of Zn.
The influence of multi-microalloying in the Al-Cu-Li-Zr alloys, such as Mg+Ag+Zn, Mg +Mn+Zn, Mg+Ag+Mn and Mg+Ag+Zn+Mn are firstly systema- tically investigated in this study, which is illustrated in Table 4. The results show that very high yield strength, higher than 600 MPa, can be obtained by multi-microalloying. The alloy X104 with Mg+Mn+Zn exhibits a good combination of high strength and acceptable plasticity. The alloy X105 has the same strength with X104, but has lower elongation and higher cost because of Ag addition. The alloy X106 just has little strength increment compared with X104 and X105.
Table 5 shows the effects of microalloying elements of Mg, Ag, Mn and Zn individually or combined. Obviously, the alloy containing Mg+Zn has the highest yield strength increment, 32 MPa, than that calculated from the individual microalloying element, which means that the extra-effects of combined additions of Mg+Zn can be obtained contrasted with the individual addition of Mg or Zn. When considering the difference of experimental Δσ0.2 and calculated 
in Table 5, it is found that the element Mn can also bring some extent strengthening effects on the alloys, but it has nothing to do with the other microalloying elements present or not. The combined additions of Mg+Ag+Zn and Mg+Ag+ Zn+Mn can not bring the ideal strengthening effects because of the negative difference of experimental Δσ0.2 and calculated in Table 5.
Table 3 Compositions and properties of alloys for investigating effects of Zn
Table 4 Compositions and properties of alloys for investigating multi-microalloying effects
Table 5 Effects of microalloying elements on yield strength
3.2 Developing new Al-Li alloy
Based on the above results, a new alloy named 2A97 with a nominal composition of Al-3.7Cu-1.5Li- 0.4Mg-0.3Zn-0.4Mn-1.12Zr is designed and produced for aerospace applications. The density of the alloy is 2.67 g/cm3, representing a decrease by 4% than that of 2124 alloy and by 6% compared with 7050 alloy. Stress corrosion cracking(SCC) susceptibility was measured in a 3.5%NaCl solution. No specimen failures associate with SCC under stress levels of 240 MPa extending to 40 d.
Typical tensile properties and fracture toughness for 30 mm-thick plates are illustrated in Table 6. The fatigue crack growth propagation(FCGP) is shown in Fig.3. The results show that the new Al-Li alloy has better properties such as strength, toughness and FCGP than 7050-T76 alloy. Thermal exposures at temperature up to 150 ℃ for 100 h do not appear to have detrimental influence on the tensile properties, with the transactional tensile strength of 540 MPa, yield strength of 470 MPa and elongation of 7%. These results show that the new alloy possesses very promising properties for aerospace applications.
Microstructural examinations were performed in order to determine the strengthening precipitates and grain boundary features. Because the T1-phase has a {111} habit plane[11-14], one of the best observing direction is
Fig.3 FCGP of 7050 and Al-Li alloys
<112>Al, from which only one variant of T1-phase is visible. A TEM dark field(DF) image and a bright field(BF) image of T1 phase are shown in Fig.4. Selected-area electron diffraction(SAED) patterns are shown in Fig.5. The majority of microscopic features are noted that: 1) The microstructure is mainly dominated by unrecrystallized pancake-like grain; 2) The major strengthening phase identified is T1 precipitates with less θ′ and S′; 3) Two types of dispersoid particles are also observed, including Al6Mn and Al3Zr.
Table 6 Tensile properties and fracture toughness of new alloy plate
Fig.4 BF (a) and DF (b) images of T1 phase under B=[112]Al
Fig.5 SAED patterns of new Al-Li alloy: (a) B=[001]Al; (b) B=[110]Al; (c) B=[111]Al; (d) B=[112]Al; ● Come from T1 phases, 〇 Come from LI2 phases
4 Conclusions
1) Some strengthening effects on the Al-Cu-Li alloys are brought when the microalloying elements such Mg, Ag or Mg+Ag are added. The strengthening effect of Mn on the alloys has nothing to do with the other microalloying elements present or not.
2) Though individual addition of Zn has no strengthening effects on the Al-Cu-Li alloys, Mg+Zn additions can bring significant strengthening effects.
3) Based on the significant strengthening effects of Mg+Zn+Mn additions, a new alloy has been developed with nominal composition of Al-3.7Cu-1.5Li-0.4Mg- 0.3Zn-0.4Mn-1.12Zr, which possesses high strength and high tolerance-resistance properties for aerospace applications.
4) The transmission electron microscope analysis results show that the new Al-Li alloy is strengthened primarily by T1 phase with lesser θ′ and S′.
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(Edited by YANG Bing)
Foundation item: Project(2005CB623705) supported by the National Key Basic Research Program of China
Corresponding author: YANG Shou-jie; Tel: +86-10-62496407; E-mail: yangshoujie@sohu.com
